We port the build system to Dune. This setup will integrate nicely
with Coq 8.10, but until this arrives it also provides a few
advantages:

- better `.merlin` files, automatically generated,
- clean build tree: all the artifacts go under _build,
- modular organization of the development: we introduce a separate
  library for the JS Coq package manager.

Note indeed that now all the build artifacts do live in
`_build/default`; `make links` will allow you to serve pages in-tree.

Pending:

- fully split the libmngr build to its own repository.
- libjs: implement recursive scan / get closer to Dune.